A study on the synthesis, longitudinal optical phonon–plasmon coupling and electronic structure of Al doped ZnS nanorods

Abstract

First principles density functional theory (DFT) calculations were employed to study the structural and electronic properties of pure and Al doped ZnS nanorods. The wurtzite-type pure and Al doped ZnS nanorods were synthesized via a hydrothermal method at 175 °C using ethylenediamine as a solvent and complexing ligand. The TEM results reveal that the diameter of the nanorods decreases from 12 to 15 nm with increasing Al doping. It was observed from the UV-Vis spectra of the samples that the energy band gap decreases with Al content. In the Fourier transform infrared (FT-IR) spectra, a noticed splitting of the ZnS peak at 633 cm⁻¹ into three peaks at 489, 613 and 744 cm⁻¹ for the Al doped ZnS nanorods indicates that the Al atom can be partially substituted in the zinc as Al–S–Zn. The atomic concentrations obtained from XPS and EDS are consistent, with XPS demonstrating the successful doping of Al into ZnS. From the XPS, it was observed that the binding energy (BE) values were shifted toward the lower BE side for increasing aluminium content. Raman spectra were obtained for the pure and Al doped ZnS nanorods which exhibit first-order phonon modes at 344.26 and 346.78 for the A₁/E₁ longitudinal optical phonons, and the bands at 251.14 and 250 cm⁻¹ reveal longitudinal optical phonon–plasmon coupled (LOPC) modes. It was observed that the acceptor level hybridization in Al doped ZnS nanorods occurs due to the overlap of the Al 3s and S 3p states. It was confirmed from XPS and DOS that sulphur vacancies were created due to the doping of Al into the ZnS lattice. It was observed from the band structure of the Al doped ZnS nanorods that the CB and VB bands were almost overlapped in the Brillouin zone at the G-point.